The present invention relates to a head for an impact type of dot line printer, to perform dot matrix printing of characters, graphics, etc.
In general, an impact type of dot line printer comprises a plurality of narrow rod-shaped members which will be referred to in the following as printer wires, for forming dots on a printing paper, arrayed in a line adjacent to a printing paper which is passed over a platen, as disclosed in a U.S. Pat. No. 3,941,051. These are controlled by electrical signals to be selectively driven into impact with the printer paper, acting through an ink ribbon, to produce a desired pattern of dots on the paper. Each of these printer wires is usually mounted on the free end of a corresponding one of an array of leaf springs, whose other end is fixedly attached to an elongated frame. A corresponding array of magnetic coils formed on coil cores are also provided, together with permanent magnets, arranged such that a magnetic flux circuit path is formed acting on each of the free ends of the leaf springs to hold that end normally in contact against the tip of a corresponding coil core by magnetic attraction, i.e. each spring is held in a state of tension. To drive a printer wire, i.e. to cause that printer wire to fly away from the corresponding coil core towards impact with the printer paper, an electrical current is passed momentarily through the corresponding coil, to thereby momentarily cancel the force of magnetic attraction.
In order to attain a high speed of printing, together with an acceptable level of pattern definition with such a printer, it is necessary to use a high density of printer wires, i.e. these must be arrayed in line as closely adjacent to one another as possible. Due to this, it is desirable to simplify the mechanical configuration of the printer head as far as possible, in order to achieve a practicable level of manufacturing cost and component assembly time. For this reason, with prior art printer heads for such dot line printers, the frame upon which the leaf springs are mounted is generally utilized as a common magnetic member for all of the magnetic flux circuit paths of the leaf springs, i.e. the frame must be formed of a magnetically permeable material, and in addition each leaf spring forms part of a magnetic flux circuit path, so that the leaf springs must also be formed of a magnetically permeable material. In the case of the frame, this has the disadvantage that it will generally be necessary to provide additional members constituting the frame (other than the main protion formed of a magnetically permeable material) having sufficient mechanical strength for the attachment of other members to the frame. Use of such a composite frame can result in bending distortion resulting from temperature increase. In the case of the leaf springs, the most suitable material from the aspects of optimum spring characteristics cannot be used to form the leaf springs. In addition, the cross-sectional area of each leaf spring must be held above a certian level, from considerations of the magnetic flux circuit path, so that the optimum shape of each leaf spring to provide optimum spring characteristics cannot be utilized. Due to these factors, maximum printing speed cannot be achieved with such a prior art head for an impact type of dot line printer.
Furthermore, since the frame of such a prior art printer head is a common to all of the magnetic flux circuit paths of the various leaf springs, magnetic coupling occurs between adjacent coils, due to magnetic flux passing through the frame. Thus, when a number of coils are driven simultaneously, magnetic interference between these can occur. Such magnetic interference can reduce the rate of rise of drive current pulses flowing in the coils, thereby hindering the movement of printer wires which are flying towards impact on the printer paper. This results in non-uniformity of printing density, and also to increased power consumption.
It has been proposed to reduce the effects of such magnetic interference by increasing the duration of pulses of drive current passed through the coils. However this will result in an increase in the coil temperature, producing an increased temperature of the printer head as a whole. This leads to restrictions on the time duration for which the printer can be continuously operated, i.e. leads to a reduced operating duty cycle.
In order to overcome these disadvantages of a prior art head for an impact type of dot line printer, the present applicant has disclosed a design for a printer head (in Japanese patent application No. 57-128761), in which each of the printer wires is controlled by a separate independent magnetic flux circuit path, with each of the coil cores being mounted on an individual magnetic flux member through which the magnetic flux circuit path of that coil core passes, rather than through the frame. In this way, interference between the magnetic flux circuit paths corresponding to the various leaf springs is eliminated, since the frame is not used as a common magnetic flux member and so can be formed of a suitable lightweight material. In addition, a stud formed of a magnetically permeable material is fixedly attached to the free end of each of the leaf springs, with the magnetic flux circuit path passing through this stud rather than through the leaf spring. In this way, the material used to form the leaf springs can be selected to provide optimum spring characteristics, with no regard for the magnetic properties of the material or the effects of cross-sectional area upon the magnetic flux circuit path.
However this method presents the following disadvantage if it is desired to provide a very high density of printer wires. Since each of the coil cores is mounted on a separate magnetic flux path member, the size of each of these magnetic flux path members must be made very small if the printer wire density is made high. As a result, these magnetic flux path members do not provide a sufficient degree of support stability for the coil cores mounted thereon. Due to this, when machining of the end faces of the coil cores is performed during manufacture (i.e. the end faces onto which the free ends of the leaf springs are to be held attracted), it is found that vibration of the coil cores is produced. This vibration causes the accuracy of this machining to be reduced, and since these end faces of the coil cores must be machined to a very high degree if accuracy in order to attain correct operation of the printer head after assembly, this is a serious disadvantage. Furthermore, with a high degree of density of the printer wires, the spacing between adjacent ones of these magnetic flux path members mounting the coil cores must be made small, and this can result in magnetic coupling between adjacent magnetic flux circuit paths, producing the type of magnetic interference described above, with the problems which arise from this.